1. Field of the Invention
The present invention relates to microwave amplification tubes, such as a traveling wave tube (TWT) or klystron, and, more particularly, to a coupled cavity microwave electron tube that produces an inverted slot mode and a broadband response.
2. Description of Related Art
Microwave amplification tubes, such as TWT""s or klystrons, are well known in the art. These devices are designed so that a radio frequency (RF) signal and an electron beam are made to interact in such a way as to amplify the power of the RF signal. A coupled cavity TWT typically includes a series of tuned cavities that are linked or coupled by irises (also know as notches or slots) formed between the cavities. A microwave RF signal induced into the tube propagates through the tube, passing through each of the respective coupled cavities. A typical coupled cavity TWT may have thirty or more individual cavities coupled in this manner. Thus, the TWT appears as a folded waveguide; the meandering path that the RF signal takes as it passes through the coupled cavities of the tube reduces the effective speed of the signal causing the electron beam to operate effectively upon the signal. Thus, the reduced velocity waveform produced by a coupled cavity tube of this type is known as a xe2x80x9cslow wave.xe2x80x9d
Each of the cavities is linked further by an electron beam tunnel that extends the length of the tube and through which an electron beam is projected. The electron beam is guided by magnetic fields which are induced into the beam tunnel region; the folded waveguide guides the RF signal periodically back and forth across the drifting electron beam. Thus, the electron beam interacts with the RF signal as it travels through the tube to produce the desired amplification by transferring energy from the electron beam to the RF wave.
The magnetic fields that are induced into the tunnel region are obtained from flux lines that flow radially through polepieces from magnets lying outside the tube region. The polepiece is typically made of permanent magnetic material, which channels the magnetic flux to the beam tunnel. This type of electron beam focusing is known as Periodic Permanent Magnet (PPM) focusing.
Klystrons are similar to coupled cavity TWTs in that they can comprise a number of cavities through which an electron beam is projected. The klystron amplifies the modulation on the electron beam to produce a highly bunched beam containing an RF current. A klystron differs from a coupled cavity TWT in that the klystron cavities are not generally coupled. A portion of the klystron cavities may be coupled, however, so that more than one cavity can interact with the electron beam. This particular type of klystron is known as an extended interaction klystron (EIK).
For a coupled cavity circuit, the bandwidth over which the amplification of the resulting RF output signal occurs is typically controlled by altering the dimensions of the cavities and irises and the power of the RF output signal is typically controlled by altering the voltage and current characteristics of the electron beam. More specifically, for a coupled cavity circuit to propagate higher frequencies, the cavity size for the circuit has to be smaller. For a circuit to propagate more frequencies, the iris size has to be larger.
There are generally two frequency bands of interest in which propagation can occur. The lower frequency, first passband is referred to as the xe2x80x9ccavity passbandxe2x80x9d because its characteristics are controlled largely by the cavity resonance condition. The higher frequency, second passband is referred to as the xe2x80x9ciris passbandxe2x80x9d and its characteristics are controlled mainly by the iris resonance condition. Normally, the second space harmonic (between xcfx80 and 2xcfx80 of the dispersion curve) of the first passband (or cavity passband) is used for interaction with the electron beam. As the length of the iris increases, the cavity resonance condition (usually appearing at the 2xcfx80 point on the lower first passband of the dispersion curves) changes position with the iris resonance condition, which appears at the 2xcfx80 point on the upper second passband. When this passband mode inversion occurs (i.e., cavity passband and iris passband trading relative positions), it provides the advantage of preventing drive-induced oscillations. Thus, no special oscillation suppression techniques are required. It should be noted that the mechanism of exciting the oscillations with a decelerating beam crossing a cavity resonance point is well known.
Unfortunately, to produce this passband mode inversion (also know as inverted slot mode), the iris length is usually to such an extent that it wraps around the electron beam tunnel. This has the disadvantage of introducing transverse magnetic fields when the iris lies in an iron polepiece. Furthermore, a significant problem with RF amplification tubes is the efficient removal of heat. As the electron beam drifts through the tube cavities, heat energy (resulting from stray electrons intercepting the tunnel walls) must be removed from the tube to prevent reluctance changes in the magnetic material, thermal deformation of the cavity surfaces, or melting of the tunnel wall. The excessive iris length and corresponding reduction in the amount of metal results in a longer heat flow path around the iris. Thus, the ability to remove heat is reduced significantly along with the overall coupled cavity circuit""s thermal ruggedness.
Accordingly, it would be desirable to provide a coupled cavity circuit having an iris that produces the passband mode inversion without the excessive iris length. Also, it would be desirable for the coupled cavity circuit to have a broadband frequency response (i.e., many and higher frequencies) while preventing drive-induced oscillations so that no special oscillation suppression techniques are required. Furthermore, it would be desirable for such a coupled cavity circuit to offer a significant increase in the amount of metal provided around the electron beam tunnel such that a passband mode inversion occurs without an increase in transverse magnetic fields or degradation in thermal ruggedness.
In addition, a coupled cavity circuit that propagates higher and more frequencies at higher power would be advantageous. As mentioned, typically for a coupled cavity circuit to propagate higher frequencies, the cavity size for the circuit has to be smaller. Similarly, for a circuit to propagate more frequencies, the iris size has to be larger. But, for a coupled cavity circuit to increase output power, the cavity size must be larger and the iris size has to be smaller since a more thermally rugged circuit is needed to handle the higher power. A circuit having a larger cavity and a smaller iris is more thermally rugged.
Accordingly, for high power designs, it would also be desirable to provide a coupled cavity circuit that propagates higher frequencies without decreasing (or narrowing) the cavity size and propagates more frequencies without increasing the iris size. It would further be desirable for such a circuit to have outputs with flat frequency responses (i.e., less distortions).
In accordance with the teachings of the present invention, a coupled cavity circuit is provided with an iris that produces passband mode inversion such that the iris mode passband is at a lower frequency than the cavity mode passband. In addition, the coupled cavity circuit also provides broadband frequency response while preventing drive-induced oscillations so that no lossy material is required within the coupled cavity circuit. Furthermore, the coupled cavity circuit provides these advantages without requiring an excessive iris length and, thus, avoids any severe increase in transverse magnetic fields or degradation in thermal ruggedness.
In an embodiment of the present invention, a microwave electron tube, such as a traveling wave tube or an extended interaction klystron, comprises an electron gun for emitting an electron beam through an electron beam tunnel to a collector that collects the electrons from the electron beam. A slow wave structure is disposed along the electron beam path and defines an electromagnetic path along which an electromagnetic signal interacts with the electron beam. The slow wave structure has at least one coupled cavity circuit comprising at least one iris disposed between a first cavity and a second cavity for coupling the electromagnetic signal between the first cavity and the second cavity. The iris is disposed between the electron beam tunnel and a sidewall of the tube with the iris being symmetrical about a perpendicular axis of the electron beam tunnel. The iris has a center portion with a first width and flared ends with a second width that is greater than the first width. The flared ends wrap partially around the electron beam tunnel.
In a second embodiment of the present invention, the coupled cavity circuit of the slow wave structure has a rectangular shape. The iris has a rectangular central portion that extends substantially across one sidewall of the tube. The iris has flared ends that form a triangular region at each end of the central portion. The triangular regions have a hypotenuse that is adjacent to the electron beam tunnel and a side that extends part way along a sidewall of the tube that is adjacent to the one sidewall of the tube.
If there is more than one coupled cavity circuit, the irises can be in line, staggered, or on opposite sides of the tube. There can also be more than one iris per coupled cavity circuit with the irises in line or staggered from each other. The iris shape provides the inverted slot mode condition and broadband response without excessive iris length.
In a third embodiment of the present invention, a microwave electron tube is provided with an electron gun for emitting an electron beam having a predetermined voltage. The electron tube is also provided with a collector. The collector is spaced away from the electron gun. The collector is used for collecting electrons of the electron beam emitted from the electron gun. The tube is further provided with an interaction structure that defines an electromagnetic path along which an applied electromagnetic signal interacts with the electron beam. The interaction structure further comprises a plurality of cavity walls and a plurality of magnets. The plurality of cavity walls each has an aligned opening for providing an electron beam tunnel. The electron beam tunnel extends between the electron gun and the collector. The electron beam tunnel further defines an electron beam path for the electron beam. The magnets provide a magnetic flux path to the electron beam tunnel. The electromagnetic signal has a first passband and a second passband. The first passband has an upper bandedge. The second passband has first, second and third space harmonics and a lower bandedge. The interaction structure further includes respective cavities (defined therein) interconnected to provide a coupled cavity circuit. The plurality of cavity walls separating adjacent ones of the cavities. Each of the cavity walls also has an iris for coupling the electromagnetic signal therethrough. The iris and the cavity walls are dimensioned to allow the interaction structure to exhibit an inverted slot mode. The inverted slot mode comprises a cavity resonant frequency that is substantially larger than a corresponding iris cutoff frequency. The cavity resonant frequency is associated with the lower bandedge of the second passband. The iris cutoff frequency is associated with the upper bandedge of the first passband. In one embodiment, the predetermined voltage of the electron beam is determined to allow the electron beam to interact with the third space harmonic of the second passband. In another embodiment, the plurality of magnets comprise a plurality of permanent magnets. In a further embodiment, the iris and the cavity walls are dimensioned using a geometric formula to allow the interaction structure to exhibit the inverted slot mode. The geometric formula comprises:       (                                        π            2                    ⁢                      R            2                    ⁢                      ln            ⁡                          (                              R                /                A                            )                                                12          ⁢                      L            2                              +                        π          ⁢                      xe2x80x83                    ⁢                      R            2                    ⁢          Wm                          3          ⁢          GLT                      )     less than   1
wherein A represents a radius of the beam tunnel; L represents an effective length of the iris; W represents a height of the iris; R represents a radius of one of the cavities that is coupled to the iris; T represents a thickness of one of the cavity walls that is associated with the iris; G represents a gap between two of the cavity walls; and m represents a fraction of a total current circulating in one of the cavities of the coupled circuit that intercepts only one iris. In yet another embodiment, the iris comprises an iris capacitance and an iris inductance. Each of the cavity walls comprises a cavity capacitance and a cavity inductance. The iris capacitance, the iris inductance, the cavity capacitance, and the cavity inductance are selected to exhibit the inverted slot mode.
In a fourth embodiment of the present invention. An applied microwave signal is amplified by interacting with an electron beam. The electron beam is focused by using a plurality of permanent magnets. The microwave signal has a first passband and a second passband. The first passband has a upper bandedge. The second passband has first, second and third space harmonics and a lower bandedge. A cavity resonant frequency that is substantially larger than a corresponding iris cutoff frequency is exhibited during the amplification of the microwave signal. The cavity resonant frequency is associated with the lower bandedge of the second passband. The iris cutoff frequency is associated with the upper bandedge of the first passband. The electron beam interacts with the microwave signal at the third space harmonic of the second passband.
A more complete understanding of the coupled cavity circuit will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.